Particle filter and method for producing a particle filter

ABSTRACT

The invention relates to a particle filter ( 1 ) for an exhaust gas system ( 2 ), and to a method for producing a particle filter. The particle filter ( 1 ) comprises a plurality of flow channels ( 5 ), which extend from a first end face ( 6 ) towards a second end face ( 7 ) and which are separated from one another by porous channel walls ( 8 ). On the end faces ( 6, 7 ), the flow channels ( 5 ) each have mutual closing means ( 9 ) such that an exhaust gas ( 10 ) enters a flow channel ( 5 ) that is open on the first end face ( 6 ), flows through the channel wall ( 8 ), and escapes from the particle filter ( 1 ) by way of an adjacent flow channel ( 5 ) that is open on the second end face ( 7 ). In a direction of flow ( 11 ), the channel wall ( 8 ) has, in succession, the following layers: a particle filter layer ( 13 ); an intermediate layer ( 14 ) comprising a first SCR coating ( 15 ) having a first catalytic activity ( 16 ); a second SCR coating ( 18 ) having a second catalytic activity ( 19 ), wherein the second catalytic activity ( 19 ) is different from the first catalytic activity ( 16 ).

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a U.S. national stage of application No. PCT/EP2015/070387, filed on 7 Sep. 2015, which claims priority to the German Application No. 10 2014 112 862,1 filed 8 Sep. 2014, the content of both incorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The invention relates to a particle filter and a process for producing a particle filter. The particle filter is used, in particular, as exhaust gas treatment unit in an exhaust gas system, preferably in an exhaust gas system of a diesel internal combustion engine of a motor vehicle.

2. Related Art

Particle filters of this type are formed from a porous, optionally extruded, structure, for example of the honeycomb structure type. The shape of this honeycomb structure is not subject to any particular restrictions. However, a circle, an ellipse or an oval, for example, can serve as outer cross-sectional shape of the honeycomb structure. The cross-sectional shape of the flow channels is likewise not subject to any restrictions, but an angular cross-sectional shape is normally preferred, for example of the type of a triangle, a quadrilateral, a hexagon or the like. The cell density for the flow channels can likewise be varied within wide limits, and preference is given, for example, to the shape having a flow channel density in the range from 50 to 400 cells per square inch (7.8 to 62 cells per square centimeter). The porous channel walls can, for example, be formed by ceramic. Silicon carbide or metal-silicon, and silicon carbide, for example, have been found to be suitable. If the ceramic has a metal-silicon and silicon carbide as main crystal phase, the Si content defined by Si (Si+SIC) is preferably from 5 to 50% by mass, preferably from 10 to 40% by mass.

Such particle filters are normally referred to as “wall-flow filters” because they force at least a major part of the exhaust gas stream through the porous channel walls. For this purpose, alternate closure of the adjacent flow channels of the particle filter at the two end faces is known. A “closure” can also be configured with a significantly smaller passage opening compared to the channel cross section, in particular in the case of selected or all channels, at the second (exhaust gas outflow end) end face, so that a bypass is made possible (and thus blocking of the particle filter when correct flow cannot occur through the channel wall as a result of the particle loading). A substream of the exhaust gas thus flows into a first flow channel, which is open at the exhaust gas outflow end and is forced by the closure at the end of this flow channel to flow (at least mostly during normal operation of the particle filter) through the porous channel wall into an adjacent flow channel and flow out through this open end on the exhaust gas exit side. During flow through the porous channel walls, particles entrained in the exhaust gas can also be embedded and optionally reduced or converted into gaseous constituents.

Such particle filters having porous walls normally have a particularly large internal surface area, so that a very large catalytically active area can be provided here for a corresponding coating in a relatively small construction volume. For this reason, the use of such particle filters has been proposed as preferred, also in combination with an SCR system. In selective catalytic reduction (SCR), the nitrogen oxides (NO, NO₂) are preferentially reduced while undesirable secondary reactions (e.g., the oxidation of sulfur oxide to sulfur trioxide) are largely suppressed. For this reaction to proceed, ammonia (NH₃) is normally used as a reducing agent that is mixed into the exhaust gas. The products of the reaction are water (H₂O) and nitrogen (N₂). Suitable catalysts employed are, for example, titanium dioxide, vanadium pentoxide and/or tungsten oxide. The use of zeolites is also possible. The SCR process is used particularly in diesel vehicles, especially in commercial vehicles, in order to decrease the pollutant emissions in respect of nitrogen oxide pollution.

The systems proposed hitherto for firstly particle reduction and secondly nitrogen oxide reduction sometimes have a very complex structure and require a large amount of construction space. In addition, it is necessary in the case of some systems for complicated conditioning of the reducing agent, for example urea or the like, to be carried out externally to the exhaust gas and/or in the exhaust gas system itself, and complete conversion can sometimes not be ensured. For this reason, provision of a barrier catalyst intended to prevent breakthrough of nitrogen oxides in the event of insufficient conversion of the reducing agent has frequently also been proposed.

SUMMARY OF THE INVENTION

In view of the problems of the prior art discussed above, it is an object of the present invention to at least partly solve these problems of the prior art. In particular, in one aspect, a particle filter that provides advantages with respect to complete conversion of the SCR reactions or the hydrolysis of added reducing agent possible in SCR systems in particular is provided. In addition, a process for producing such particle filters is proposed, so that reliable and inexpensive production is made possible.

These objects are achieved by a particle filter and by a process for producing a particle filter. In particular, the process serves to produce the particle filter of the invention. The features safe forth individually can be combined with one another in any technologically purposeful way and can be supplemented by explanatory subject matter from the description, with further variants of the invention being indicated.

According to a first aspect of the present invention a particle filter for an exhaust gas system, in particular for a motor vehicle having a diesel internal combustion engine, is provided. The particle filter comprises a plurality of flow channels extending from a first end face through to a second end face and being separated from one another by porous channel walls. The flow channels each have a closure alternately at the end faces so that an exhaust gas enters a flow channel open at the first end face, flows through the channel wall and flows out from the particle filter through an adjacent flow channel open at the second end face. The channel wall has at least the following layers in succession in a flow direction of the exhaust gas:

-   -   a particle filter layer having a porosity of from 60 to 90%,         preferably from 80 to 90%, and an average pore size of from 2 to         10 μm [micron], preferably from 3 to 5 μm on the exhaust gas         entry side;     -   an intermediate layer having a porosity of from 40 to 60%,         preferably from 50 to 60%, and an average pore size of from 10         to 20 μm [micron] , preferably from 14 to 16 μm, where the         intermediate layer comprises a first SCR coating having a first         catalytic activity;     -   a second SCR coating having a second catalytic activity, where         the second catalytic activity is different from the first         catalytic activity, on the exhaust gas exit side.

The particle filter layer serves, in particular, to filter solid particles (soot or the like) out of the exhaust gas stream. These particles are held back by the small pores in the particle filter layer, so that a major part of the particles, preferably at least 80% by mass, cannot pass through into the intermediate layer. The particle filter layer can be regenerated at time intervals that can be determined freely or continuously (CRT), so that particles are converted by NO₂ present in the exhaust gas or by thermal oxidation.

In particular, in another aspect the particle filter layer additionally comprises a hydrolysis coating, so that a reducing agent precursor (e.g., urea/water solution) added upstream of the particle filter is at least partly converted into a reducing agent (e.g., ammonia) during passage through the particle filter layer.

In another aspect, the particle filter layer is, in particular, applied in the form of a coating to the exhaust gas entry side of the porous channel walls, so that there is a sharp separation between the particle filter layer and the intermediate layer. The particle filter layer consists, in particular, at least of a

-   -   washcoat (Al₂O₃),     -   which optionally additionally comprises a hydrolysis coating,         wherein the hydrolysis coating comprises, in particular,         -   titanium dioxide or         -   titanium oxide-supported tungsten dioxide catalysts and             vanadium-tungsten oxide catalysts.

In another aspect, the thickness of the particle filter layer is, in particular, from 30 to 150 μm, preferably from 50 to 100 μm.

In another aspect, the intermediate layer arranged downstream of the particle filter layer has a first SCR coating having a first activity. The first activity of the first SCR coating in the intermediate layer can also be indicated by a density of the catalytically active material or by the average distance between catalytically active sites.

In particular, in another aspect, the particle filter has a porous channel wall which consists, at least in the region of the intermediate layer before a coating with the first SCR coating, exclusively of a ceramic base material (see introduction). Only in a coating step carried out separately is the first SCR coating applied, so that the above-described properties of the intermediate layer in respect of porosity and pore size are then present.

In particular,, the thickness of the intermediate layer is from 200 to 500 μm, preferably from 200 to 400 μm.

In another aspect, the particle filter additionally has a second SCR coating on the side of the intermediate layer opposite the particle filter layer, where the second catalytic activity is different from, the first catalytic activity, i.e., in terms of the respective density or type of the catalytically active material or the respective average distance between catalytically active sites.

In particular, the catalytic activity of the first and second SCR coatings is set via the proportion of the catalytically active materials in the coating, i.e., for example, titanium dioxides tungsten dioxide, vanadium pentoxide or zeolite.

In particular, the thickness of the second SCR coating is from 10 to 200 μm, preferably from 10 to 50 μm.

The arrangement of the particle filter layer upstream of the intermediate layer and upstream of the second SCR coating ensures that soot particles can be converted by NO₂ present in the exhaust gas. The NO₂ present in the exhaust gas is thus available, especially at low temperatures, exclusively for the conversion of soot particles. Only after flowing through the particle filter layer is NO₂ (and other NO_(x) compounds) converted in the SCR-coated intermediate layer and in the second SCR coating according to the following reactions:

4NH₃+4NO+O₂→4N₂+6H₂O (“Standard SCR”)

2NH₃+NO+NO₂→2N₂+3H₂O (“Fast SCR”)

4NH₃+3NO₂→3.5N₂+6H₂O (“NO₂ SCR”)

In particular, the first SCR coating in the intermediate layer has a lower first catalytic activity than the second catalytic activity of the second SCR coating.

In particular, the porous intermediate layer makes it possible for nitrogen oxides to be temporarily stored and gradually released again into the exhaust gas stream. The second SCR coating arranged downstream of the intermediate layer ensures, in particular in the case of a relatively high catalytic activity that a virtually complete conversion of the nitrogen oxides occurs. The multistage coating of the particle filter makes it possible for a separation of particle deposition and particle conversion, on the one hand, and nitrogen oxide reduction, on the other hand, to be ensured. Thus, the NO₂ present in the exhaust gas can be utilized effectively for the continuous regeneration of the particle filter, with the further conversion of the nitrogen oxides then still, present occurring directly downstream of the particle filter layer in the same channel wall of the particle filter. Furthermore, a space-saving arrangement of particle filter and SCR catalyst, optionally also hydrolysis catalyst, can be created, with large surface areas being simultaneously provided for particle attachment and catalytic conversion. In addition, the e.g., ceramic porous channel wall of a conventional particle filter can now be used simultaneously for hydrolysis, particle filtration and nitrogen oxide temporary storage/conversion. The required SCR activity and temporary storage capacity can be set precisely by the different first and second SCR coatings, so that effective utilization of the catalytically active materials is possible.

In another aspect, a process for producing a particle filter, in particular for producing a particle filter according to the invention, is proposed. The process comprises at least the following steps:

a) provision of a particle filter having a plurality of flow channels extending from a first end face to a second end face and being separated from one another by porous channel walls;

b) alternate arrangement of closures in the flow channels at the end faces;

c) coating of the porous channel wall with a first SCR coating that has a first catalytic activity;

d) coating of the channel walls of the flow channels that are open at the second end face with a second SCR coating having a second catalytic activity, where the second catalytic activity is different from the first catalytic activity;

e) arrangement of a particle filter layer on channel walls of the flow channels that are open at the first end face, where the particle filter layer has a porosity of from 5 to 50% and an average pore size of from 5 to 15 μm [micron], preferably from 5 to 10 μm.

It is expressly pointed out that the information given in respect of the particle filter is also applicable to the process and vice versa. This applies particularly for information regarding the particle filter layer, the first and second SCR coatings and the intermediate layer.

In a preferred configuration, at least the channel walls as per step a) and the particle filter layer as per step a) are produced by a printing process. In particular, it is possible to use a three-dimensional printing process by which a particle filter and, in particular, the channel walls can be produced in layers. A printing process also enables the different properties of the individual layers (particle filter layer, intermediate layer) to be separated sharply from one another.

In an advantageous embodiment, the particle filter layer as per step e) is applied by a coating process. In particular, the closures as per step b) are arranged before the step e), so that coating can be carried out via the exhaust gas entry end of the particle filter, via the channels of the particle filter which are open at the exhaust gas entry end.

In particular, the particle filter layer as per step e) is applied at least after step c). It can in this way be ensured that catalytically active materials of the first SCR coating do not penetrate into the particle filter layer. A sharp separation of first SCR coating and particle filter layer is thus possible.

In particular, the second catalytic activity of the second SCR coating is greater than the first, catalytic activity of the first SCR coating.

In particular, the first SCR coating used for step c) has a first viscosity and the second SCR coating used for step d) has a second viscosity, where: first viscosity<second viscosity.

The low viscosity of the first SCR coating allows uniform, and permeating, coating of the intermediate layer, i.e., of the porous channel wall, with catalytically active materials. The higher viscosity of the second SCR coating ensures that the second SCR coating does not mix with the first SCR coating, or covers the latter in the pores of the intermediate layer. In particular, it is ensured in this way that even here, between first and second SCR coatings, there is a sharp spatial separation. In particular, the second SCR coating is thus applied only to the exhaust gas exit side of the porous channel walls, with the second SCR coating not penetrating, in particular, into the intermediate layer, or into the porous channel wall. The invention is also directed to a motor vehicle having an internal combustion engine and an exhaust gas system, wherein the exhaust gas system comprises a particle filter according to the invention or a particle filter produced by the process of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and also the wide technical field are explained in more detail below with reference to the figures. It may be pointed out that the figures and in particular the size ratios depicted in the figures are purely schematic. In the drawings:

FIG. 1: shows a motor vehicle having an exhaust gas system;

FIG. 2: shows a section of a particle filter;

FIG. 3: shows a detail of FIG. 2;

FIG. 4: shows a process step a);

FIG. 5: shows a process step b);

FIG. 6: shows a process step c);

FIG. 7: shows a process step d); and

FIG. 8: shows a process step e).

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

FIG. 1 shows a motor vehicle 3 having an internal combustion engine 4 and an exhaust gas system 2. The exhaust gas system 2 comprises an addition unit 22 for a reducing agent or a reducing agent precursor, which can be taken from a tank 23. A particle filter 1 is arranged downstream of the addition unit 22. An exhaust gas 10 flows through the exhaust gas system 2 from the internal combustion engine through to the particular filter The exhaust gas 10 enters the particle filter 1 via a first end face 6 and leaves it again via a second end face 7.

FIG. 2 shows a section of the particle filter 1 as shown in FIG. 1. The particle filter comprises a plurality of flow channels 5 which extend, parallel to one another and separated from one another by porous channel walls 8, from the first end face 6 to the second end face 7. Closures 9 are arranged alternately in the flow channels 5 at the end faces 6 and 7. The exhaust gas 10 enters the open flow channels 5 via the first end face 6 and is forced by closure 9 to flow through the channel walls 8 in the flow direction 11. Thus, the flow channels 5, which are open at the first, end face 6 form the exhaust gas entry side 12 and the flow channels 5, which are open at the second end face 7 form the exhaust gas exit side 17.

FIG. 3 shows a detail III of FIG. 2. A channel wall 8 extends from the first end face 6 through to the second end face 7. An exhaust gas 10 flows in the flow direction 11 through the porous channel wall 8. A particle filter layer 13 is arranged on the channel wall 8 on the exhaust gas entry side 12. Downstream of the particle filter layer 13, an intermediate layer 14 which comprises a first SCR coating 15 having a first catalytic activity 16 is present in the porous channel wall 8. Downstream of the intermediate layer 14, on the exhaust gas exit, side 17 of the channel wall 8, there is a second SCR coating 18 having a second catalytic activity 19.

It can be seen in FIG. 3 that the second SCR coating 18 has been applied only after arrangement of the closures 9 and that the particle filter layer 13 has been applied before arrangement of the closures 9.

Particle filter layer 13, intermediate layer 14 and second SCR coating 18, each having different thicknesses 24, form the channel wall 8 in the finished particle filter 1.

FIG. 4 shows process step a), i.e., the still uncoated channel walls B that form the flow channels 5.

FIG. 5 shows process step b), in which closures 9 are arranged alternately, so that the flow direction 11 is then prescribed by the channel walls 8.

FIG. 8 shows process step c), in which the porous channel wall 8 is provided with a first SCR coating 15 f which has a first viscosity 20, so as to form an intermediate layer 14. This process step can, in particular, also be carried out before process step b).

FIG. 7 shows process step d), in which the second SCR coating 18, which has a second viscosity 21, is arranged on the intermediate layer 14 on the exhaust gas exit side 1 of the channel wall 8, The second viscosity 21 is greater than the first viscosity 20 of the first SCR coating 15, so that it is not possible for the second SCR coating 18 to penetrate into the intermediate layer 14.

FIG. 8 shows process step e), in which the particle filter layer 13 is arranged on the intermediate layer 14 on the exhaust gas entry side 12 of the channel wall 8.

It may be pointed out as a precaution that the combinations of technical features shown in the figures are not absolutely necessary in general. Thus, technical features of one figure can be combined with other technical features of a further figure and/or of the general description. Anything different only applies when the combination of features has been explicitly stated here and/or a person skilled in the art sees that the basic functions of the apparatus/of the process otherwise can no longer be performed.

Thus, while there have been shown and described and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described, in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated, by the scope of the claims appended hereto.

LIST OF REFERENCE NUMERALS

-   1 Particle Filter -   2 Exhaust gas system -   3 Motor vehicle -   4 Internal combustion engine -   5 Flow channel -   6 First end face -   7 Second end face -   8 Channel wall -   9 Closure -   10 Exhaust gas -   11 Flow direction -   12 Exhaust gas entry side -   13 Particle filter layer -   14 Intermediate layer -   15 First SCR coating -   16 First catalytic activity -   17 Exhaust gas exit side -   18 Second SCR coating -   19 Second catalytic activity -   20 First viscosity -   21 Second viscosity -   22 Addition unit -   23 Tank -   24 Thickness 

1-9. (canceled)
 10. A particle filter (1) for an exhaust gas system (2), the particle filter (1) comprising: a first end face (6); a second end face (7); a plurality of porous channel walls (8); and a plurality of Sow channels (5) extending from the first end face (6), at which an exhaust gas (10) enters, through to the second end face (7), the flow channels (5) being separated from one another by respective ones of the porous channel walls (8), the flow channels (5) each having a closure (9) arranged alternately at the first and second end laces (6, 7) so that the exhaust gas (10) entering a flow channel (5) that is open at the first end face (6) flows through the channel wall (8) and flows out from the particle filter (1) through an adjacent flow channel (5) that is open at the second end face (7), each channel wall (8) having at least the following layers in succession in a flow direction (11): a particle filter layer (13) having a porosity of from 5 to 50% and an average pore size, of from 5 to 15 μm on the exhaust gas entry side (12); an intermediate layer (14) having a porosity of from 55 to 95% and an average pore size of from 15 to 100 μm, the intermediate layer (14) comprising a first selective catalytic reduction (SCR) coating (15) having a first catalytic activity (16); and a second SCR coating (18) having a second catalytic activity (19), different from the first catalytic activity (16), arranged on an exhaust gas exit side (17).
 11. The particle filter (1) as claimed in claim 10, wherein the second catalytic activity (19) is greater than the first catalytic activity (16).
 12. A process for producing a particle filter (1), the process comprising: providing a plurality of flow channels (5) each extending from a first end face (6) to a second end face (7) and being separated from one another by porous channel walls (8); alternately arranging closures (9) in the flow channels (5) at the first and second end faces (6, 7); coating the porous channel wall (8) with a first selective catalytic reduction (SCR) coating (15) having a first catalytic activity (16); coating the channel walls (8) of the flow channels (5) that are open at the second end face (7) with a second SCR coating (18) having a second catalytic activity (19), the second catalytic activity (19) being different from the first catalytic activity (16); arranging a particle filter layer (13) on channel walls (8) of the flow channels (5) that are open at the first end face (6), the particle filter layer (13) having a porosity of from 5 to 50% and an average pore size of from 5 to 15 μm.
 13. The process as claimed in claim 12, wherein at least fee channel walls (8) and the particle filter layer (13) are produced by a printing process.
 14. The process as claimed in claim 12, wherein the particle filter layer (13) is applied by a coating process.
 15. The process as claimed in claim 12, wherein the second catalytic activity (19) is greater than the first catalytic activity (16).
 16. The process as claimed in claim 12, wherein the first SCR coating (15) has a first viscosity (20) and fee second SCR coating (18) has a second viscosity (21), wherein the first viscosity (20) is less than the second viscosity (21).
 17. A motor vehicle (3) comprising: an internal combustion engine (4); and an exhaust gas system (2), the exhaust gas system (2) having a particle filter (1) as claimed in claim
 10. 